1. Field of the Invention
The present invention relates to a reversible optical information recording medium such as an optical disk which is designed for a high signal density and quick recording, reproducing, erasing and rewriting of optical information by means of a laser beam.
2. Description of the Prior Art
Technics have been known for high-density recording and reproducing of information by means of a laser beam, and are being vigorously applied to various systems such as document filing systems, still picture filing systems and so forth. At the same time, reports are being made concerning the studies and developments of rewritable recording systems.
On the other hand, recording mediums have been proposed in which repetitional recording and erasing of information are effected through changing optical charactersitics such as the refractive index, extinction coefficient and so forth by causing a reversible structural change in the material of a recording thin film as the recording medium by means of high-density energy of a laser beam. One of these recording mediums makes use of a reversible phase change of Te.
For instance, the specification of U.S. Pat. No. 3,530,441 discloses a Te-based amorphous thin film containing no oxygen, e.g., Te.sub.81 Ge.sub.15 Sb.sub.2 S.sub.2. In this recording medium, the phase change between the amorphous phase and the crystalline phase of Te is facilitated by the addition of additional elements such as Ge, Sb and S. This recording medium, however, has not been put into practical use satisfactorily because of insufficient recording sensitivity and small resistance to oxidation by moisture.
Attempts for improving the moisture-proof property has been made by using a thin film made of an alloy of Te-O system.
In this Te-O system alloy, fine grains of Te of less than 20A in size are dispersed in the TeO.sub.2 matrix or, alternatively, Te and TeO.sub.2 are mixed with each other in an almost amorphous state such that no peak can be found through an X-ray diffraction. It has been reported that a reversible recording and erasing is possible with a medium made of an alloy system formed by adding S and Se to the Te-O system alloy as shown in U.S. Pat. No. 4,278,734 or with a medium of an alloy system formed by adding Sn, Ge, In, Sb and/or Bi to the Te-O system alloy as shown in Japanese Patent Laid-Open No. 185048/1984.
When these systems are irradiated for a short period of time by a laser beam of a high energy density, the temperature of the area irradiated with the laser beam is raised abruptly to a level above the melting point so that the Te atoms are made random. After the irradiation, this area is cooled quickly and is frozen while remaining the Te atoms random, thus recording an information in amorphous state. When this area is irradiated with a laser beam of a comparactively low energy density for a comparatively long period of time, the irradiated area is heated and maintained over the crystallization temperature for a while, so that Te atoms resumes the initial ordering such as to be crystallized thus erasing the recorded information. The additional elements mentioned before serve to stabilize the amorphous state of Te and to control the crystallization speed, upon bonding to Te. Among these elements, S and Se contribute mainly to the stabilization of the amorphous state of Te and does not produce any other effect. Thus, the alloy system containing S or Se requires a long time for crystallization, i.e., for erasing.
As to other elements such as Sn, Ge, In, Bi and Sb, minute stuides have been made and the following reports have been made.
Regarding Ge, it has been confirmed that Ge forms a strong amorphous network structure with Te such as to increase the crystallization temperature, thus controlling the thermalstability of the recording signal bit even when its amount is trace, as reported in page 87, brochure of 30th symposium of applied physics, 1983. Sn, In and Bi have such semimetallic natures as to easily form amorphous state as compared with transition metal elements such as Fe and Ni. Thus, these elements provide adantageous effects in that, during the recording by forming the amorphous state, they suppresses the growth of crystal by bonding with Te when cooled from the molten state and also in that the bondage is easily broken so that they serve as nuclides for the recovery of the crystanity during the erasing by crystallization. Thus, recording sensitivity and erasing sensitivity can be improved by suitably selecting the contents of these elements.
With these knowledges, a test optical disk was formed by using thin recording film which contains both Ge and Sn, as reported in the brochure of JAPAN DISPLAY, 1983 p. 46.
This optical disk permits an erasing of old information and recording of new information in real time and exhibits a high thermal stability of recorded signal bits, but is still unsatisfactory in that it shows only low sensitivity particularly in erasing. Therefore, when this optical disk is used in combination with a laser diode now available, it is necessary to operate the laser diode almost with its full power. In consequence, a further improvement in the sensitivity is necessary.
An approach has been made to improvement in the crystallization speed from another point of view. For instance, Japanese Patent Application No. 61463/1984 proposes that the crystallization speed of Te-O system alloy can be increased by addition of Au. In this material, however, the change of the state is possible only once irreversibly from amorphous state to crystalline state and, therefore, cannot be used for repetitional recording and erasing.